The proposed Cambrian–Ordovician global Boundary stratotype and point (GSSP) in Western Newfoundland, Canada

Abstract

AbstractSections exposing the Cambrian–Ordovician Boundary interval at Broom Point in western Newfoundland have been proposed earlier for a global systemic boundary stratotype. These lie within the Cow Head Group, a late Middle Cambrian to early Middle Ordovician allochthonous unit of limestone, shale, and conglomerate deposited at the toe of the ancient continental slope and on the adjacent continental rise. Several recent studies have further investigated the stratigraphy, sedimentology, and palaeontology of the Cow Head Group and others are under way on magnetostratigraphy and chemostratigraphy. These aspects are reviewed for six key boundary sections representing proximal to distal facies: Cow Head Ledge, Broom Point South, Broom Point North, St Pauls Inlet Quarry, Martin Point, and Green Point. In particular, new data are presented from 260 conodont samples that yielded 15500 conodonts. This intense sampling has allowed the discrimination of minor hiatuses in the proximal to intermediate facies where conglomerates have eroded and cannibalized underlying strata. New conodont data from Broom Point North have lowered the base of the C. lindstromi Zone into unit 74 conglomerates, thereby making this section unsuitable as a boundary stratotype. New collections from Green Point have yielded abundant conodonts and over 9400 conodonts have been recovered from 77 samples.The conodont, graptolite, and trilobite biostratigraphy through the boundary interval is documented allowing accurate correlation between sections and more precisely revealing small hiatuses in the proximal and intermediate facies. The sequence of conodont zones is: Eoconodontus notchpeakensis, Cordylodus proavus, C. caboti, C. intermedius, C. lindstromi and C. angulatus. These can be correlated with trilobite zones established from both in situ and clast faunas from the proximal to intermediate facies and with graptolite assemblages (of Cooper 1979) especially in the intermediate to distal facies. Three new species of Cordylodus are described (C. andresi, C. hastatus and C. tortus) and the full apparatus of Iapetognathus preaengensis is illustrated.The criteria for selecting a global boundary stratotype and point (GSSP) are reviewed in terms of the Cow Head sections. The Green Point section is shown to meet, and largely surpass, the prerequisites required of a stratotype. The Green Point section is proposed to be the global boundary stratotype with the base of the Ordovician System defined at the base of unit 23, which is the base of the Broom Point Member, Green Point Formation, at a level coincident with the base of the Cordylodus lindstromi Zone. In addition to an abundant and superbly preserved conodont fauna, this section preserves the best sequence of earliest planktic graptolites through a 40 m interval; the first nematophorous graptolites (of Assemblage 1) occur in unit 25, 6.9 m above the base of the C. lindstromi Zone. This level can be readily correlated into the proximal facies where both deep and shallow water trilobites (in situ and in clasts, respectively) show the base of the C. lindstromi Zone to lie within the Symphysurina brevispicata trilobite Subzone.